Download TinyCAD script from GitHub:
https://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Modeling/mesh_tinyCADhttps://github.com/zeffii/mesh_tiny_cad
File > User Preferences > Add-ons > Install from file...
choose the zip file
Edit mode + Edge select > C
select an edge, and another one you'd like it to extend to.
Right-click to finalize selection.
W to bring up specials menu > TinyCAD
VTX > VTX | AUTO (or) E2F | Extend Edge to Face
I'm not having much luck with this script over here.
I'm not a pro at it, but I'm hoping it gets you in the right direction.
You might need to tweak it, but at least if gives something to start from.
Or file a bug-report on GitHub, if it's not doing what you need.
I've used a similar feature in drafting class, so I get what you're going for.
Hope it does better for you.